How much sub-floor ventilation do I need?

Building Regulations Part C requires 1500mm² of ventilation opening per metre of external wall perimeter. A standard 215×65mm clay airbrick provides approximately 7500mm² of free area. This means one airbrick per 5 metres of wall, but more is better for thorough cross-ventilation.

What is a telescopic cavity liner?

A telescopic liner is a plastic sleeve that extends through the cavity wall to connect the external airbrick to the sub-floor void. Without it, the airflow would enter the cavity instead of the sub-floor space. The liner extends to suit cavity widths of 50-100mm and maintains the cavity insulation integrity.

Why are my airbricks blocked?

Airbricks are commonly blocked by soil build-up against walls, paving raised above the airbrick level, render applied over them, or deliberate blocking to reduce draughts. Blocked airbricks are a major cause of sub-floor dampness, timber decay and musty odours. They should always be cleared or replaced.

Can I use mechanical sub-floor ventilation?

Yes. Mechanical sub-floor ventilation fans can be installed as an alternative or supplement to airbricks. They are particularly useful where cross-ventilation is poor (e.g. terraced houses with only front and rear walls). Fans should provide at least 3 air changes per hour in the sub-floor void.

Should airbricks be above or below DPC?

Airbricks must be positioned below the DPC (damp proof course) level but above external ground level. If the ground level is above the airbrick, excavation or a dry area (channel) should be formed to expose them. The airbrick should be at least 75mm above external ground level.

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Sub-Floor Ventilation Calculator — Airbrick Count & Spacing

Calculate the number of airbricks needed for sub-floor ventilation at 1500mm² per metre of external wall, with telescopic liner quantities for cavity walls.

External Wall Length

Total length of all external walls

Blocked/Missing Vents

Existing blocked airbricks to clear

Cavity Walls

Telescopic liners needed

Price Per Airbrick (optional)

Installed cost per airbrick

How We Calculate This

This calculator determines the number of airbricks needed based on external wall length to Part C requirements.

The formula

Required Vent Area = External Wall Length × 1500 mm²/m

Airbricks = ceil(Required Area / Airbrick Free Area)

Standard specifications

Vent requirement: 1500mm² per metre of external wall
Clay airbrick: ~7500mm² free area (215×65mm)
Typical spacing: One per 5m of wall
Telescopic liner: For cavity walls (50-100mm)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.